Let me preface this with a few disclaimers: yes, I know that not everyone gains weight when they're in a relationship, but it does happen to a great many people; also, in spite of the title, you can be a boy, a girl, or someone who falls somewhere in between who is in a relationship with a boy, a girl, or someone who falls somewhere in between. The point is, some people notice that they tend to gain more weight when they're in a relationship. It might seem weird, but it actually makes a lot of sense. It's not because you stop caring, it's not because you've “landed” your partner and don't feel the need to “put forth effort,” and it's definitely not because you're lazy.
Here's one huge reason why some people gain weight when they have a partner, especially during those tumultuous early days. If you're an emotional eater (me!), then you may find yourself snacking more while you're still in the first flush, particularly if things are brand new or in any way unsure.